Electroless displacement gold plating solution and additive for use in preparing plating solution

ABSTRACT

An object of the present invention is to provide an electroless displacement gold plating solution, an additive for use in preparing the plating solution, and a metal composite material obtained by treatment with the plating solution. The present invention provides an electroless displacement gold plating solution containing a water-soluble gold compound, a complexing agent, and a water-soluble silver compound, and optionally a water-soluble thallium compound, a water-soluble lead compound, a water-soluble copper compound or a water-soluble nickel compound, or any combination thereof. The plating solution has good stability and, even not only immediately after the preparation but also after a lapse of a certain time period from the preparation, can be used for production of a metal composite material exhibiting a even plated appearance and also having a thick gold coating film.

TECHNICAL FIELD

The present invention relates to an electroless displacement goldplating solution, an additive for the plating solution, an electrolessdisplacement gold plating method using the plating solution, and a metalcomposite material produced by the method. The present invention alsorelates to a method for stabilizing an electroless displacement goldplating solution by using the additive for the electroless displacementgold plating solution.

BACKGROUND ART

Gold plating is generally applied on the surfaces of electronic partssuch as printed wiring boards, ceramic IC packages, ITO substrates, andIC cards, because of its physical properties such as the electricconductivity of gold, solderability, and jointability bythermocompression bonding, and its chemical properties such as oxidationresistance and chemical resistance. Many of these electronic parts needgold plating on electrically isolated sites. Therefore, for goldplating, an electroplating process is not applicable, but an electrolessplating process is suitable.

Widely known conventional techniques for electroless gold platinginclude an electroless displacement gold plating process for depositinggold accompanied with dissolution of undercoat metal such as nickel andan autocatalytic gold plating process in which a reducing agent havingcatalytic activity acts on gold and makes it deposit. Now these twotypes are typical prevailing methods for electroless gold platingprocess.

In case of the electroless displacement gold plating process, theundercoat metal is displaced by gold deposition, and therefore theundercoat metal is dissolved (etched or eroded) as the gold isdeposited. Particularly, when a thick-plated gold coating film isdesired, the electroless displacement gold plating solution forthick-plating is used. In this case, the undercoat metal may severely bedissolved. This gives an adverse effect on the physical properties ofthe resulting gold coating film, such as adhesion, wire bondability,solder jointability, and solder wettability.

In order to prevent deterioration of physical properties of the coating,prior to thick electroless displacement gold plating, thin electrolessdisplacement gold plating, and an undercoat plated coating film isformed on the undercoat metal to reduce the dissolution of the undercoatmetal.

In this process, however, most of the undercoat metal surface is coveredin the thin electroless displacement gold plating. Therefore, theundercoat metal can not be sufficiently dissolved in the thickelectroless displacement gold plating, and the gold coating film canfail to reach the required thickness. Further, according to thisprocess, the plated appearance may also be uneven in most cases.

In addition, as the thick electroless displacement gold plating, whenthe autocatalytic type electroless gold plating is used in which areducing agent is made exist in a plating solution, the bath stabilityis poor, thereby causing many problems in practical use.

Accordingly, there has been a demand for development of an electrolessdisplacement gold plating process that can provide a even platedappearance, a thick-plated coating, and good adhesion.

DISCLOSURE OF INVENTION

The present invention has been made in light of the situation asdescribed above. An object of the present invention is to provide anelectroless displacement gold plating solution that can form a goldplated coating film exhibiting a even plated appearance, beingthick-plated and also having good adhesion to an undercoat metal with noadverse effect on its physical properties such as wire bondability,solder jointability and solder wettability, and to provide anelectroless displacement gold plating method using the plating solution.

Another object of the present invention is to provide an additive foruse in preparing the electroless displacement gold plating solution anda method for stabilizing an electroless displacement gold platingsolution by adding the additive thereto.

The electroless displacement gold plating solution of the presentinvention is a plating solution containing a water-soluble goldcompound, a complexing agent, and a water-soluble silver compound.Further, the plating solution may further contain a water-solublethallium compound, a water-soluble lead compound, a water-soluble coppercompound or a water-soluble nickel compound, or any combination thereof.An electroless displacement gold plating method for treating a metallicsubstrate with the electroless displacement gold plating solution, and ametal composite material having a metallic substrate with a metalcoating formed thereon by the method of the above are also within thescope of the present invention.

Additionally, the additive for use in preparing the electrolessdisplacement gold plating solution according to the present invention isconstituted as an additive containing a water-soluble silver compound, awater-soluble thallium compound, a water-soluble lead compound, awater-soluble copper compound or a water-soluble nickel compound, or anycombination thereof. The additive is added to an electrolessdisplacement gold plating solution, thereby stabilizing the platingsolution.

BEST MODE FOR CARRYING OUT THE INVENTION

An electroless displacement gold plating solution of the presentinvention contains a water-soluble gold compound, a complexing agent,and a water-soluble silver compound, which will be described in detailbelow.

The electroless displacement gold plating solution of the presentinvention is an aqueous solution containing a water-soluble goldcompound, a complexing agent, and a water-soluble silver compound. Anygrade of water can be used as water in the plating solution of thepresent invention, as far as it can achieve the object of the presentinvention. Examples of such water include but are not limited todistilled water, pure water, and ion-exchanged water. Any organicsolvent can be contained in the electroless displacement gold platingsolution of the present invention, as far as it can achieve the objectof the present invention.

As the water-soluble gold compound for use in the electrolessdisplacement gold plating solution of the present invention, anycompound can be used as far as it is a material that is water-soluble,has a gold element, and has been used as a supply source of gold in theknown gold plating solutions, but not limited thereto. Examples of thewater-soluble gold compound include but are not limited todicyanoaurates(I) such as sodium dicyanoaurate(I) and ammoniumdicyanoaurate(I); tetracyanoaurates(III) such as potassiumtetracyanoaurate(III), sodium tetracyanoaurate(III), and ammoniumtetracyanoaurate(III); gold(I) cyanide; gold(III) cyanide;dichloroaurates(I); tetrachloro auric(III) acid compounds such astetrachloro auric(III) acid and sodium tetrachloroaurate(III); goldsulfites such as ammonium gold sulfite, potassium gold sulfite, andsodium gold sulfite; and gold oxide, gold hydroxide, and alkali metalsalts thereof. Preferably, the water-soluble gold compound is potassiumdicyanoaurate(I), potassium tetracyanoaurate(III), sodiumtetrachloroaurate(III), ammonium goldsulfite, potassium goldsulfite, orsodium goldsulfite.

One type or a mixture of two or more types of the water-soluble goldcompounds may be used. The content of the water-soluble gold compound inthe electroless displacement gold plating solution of the invention isappropriately selected depending on the undercoat metal to be plated,the desired thickness of the gold coating film, the water-soluble silvercompound and the water-soluble metal compound added to the platingsolution, and the like. The water-soluble gold compound is generallycontained in the plating solution in the range of from 0.0005 to 0.05mol/liter as a gold element, preferably from 0.005 to 0.025 mol/liter,and more preferably from 0.01 to 0.02 mol/liter. When a content of thegold element in the plating solution is less than 0.0005 mol/liter,plating reaction is slow or hardly occurs. Even a gold element contentof 0.05 mol/liter or more is not economical because of less costeffectiveness.

As the complexing agent for use in the electroless displacement goldplating solution of the invention, any compound can be used as far as itis a material that is water-soluble, can form a soluble complex with thegold element, and has been used for known gold plating solutions, butnot limited thereto. The type of the complexing agent used in theinvention is appropriately selected depending on the undercoat metal tobe plated, the desired thickness of the gold coating film, water solublegold compound, the water-soluble silver compound and the water-solublemetal compound contained in the plating solution, and the like.Preferably, examples of complexing agents include polyamines and saltsthereof, animocarboxylic acids and salts thereof, oxycarboxylic acidsand salts thereof, cyclic acid imide compounds, organic phosphonic acidsand salts thereof, and inorganic phosphoric acids and salts thereof.

Examples of the polyamines include but are not limited to straight chainpolyamines such as ethylenediamine, diethylenetriamine,diethylenetetramine, and triethylenetetramine; and cyclic polyaminessuch as piperazine, imidazolizine, and pyrazolidine. Examples of thesalts thereof include but are not limited to sulfates, hydrochlorides,nitrates, and acetates.

Examples of the aminocarboxylic acids include but are not limited toglycine, iminodiacetic acid, nitrilotriacetic acid,hydroxyethylethylenediaminetriacetic acid, tetrahydroxyethylenediamine,dihydroxymethylethylenediaminediacetic acid, ethylenediaminetetraaceticacid, cyclohexane-1,2-diaminetetraacetic acid, ethylene glycoldiethyletherdiaminetetraacetic acid, ethylenediaminetetrapropionic acid,and N,N,N′,N′-tetrabis-2-(2-hydroxypropyl)ethylenediamine. The saltsthereof include but are not limited to alkali metal salts such as sodiumsalts and potassium salts, and ammonium salts.

The oxycarboxylic acids include but are not limited to tartaric acid,citric acid, gluconic acid, succinic acid, and malic acid. Further, thesalts thereof include but are not limited to alkali metal salts such assodium salts and potassium salts, and ammonium salts.

The cyclic acid imide compounds include the cyclic acid imide compoundshaving one or two nitrogen atoms in its molecular structure, includingbut not limited to succinimide, phthalic acid imide, hydantoin, and5,5-dimethylhydantoin.

The organic phosphonic acids include the compounds having each of thestructures represented by the formulae (I) to (III) each having aplurality of phosphonic acids in its molecule, and the salts thereof:

wherein X¹ is a hydrogen atom; a C₁ to C₅ alkyl group; an aryl group; anaryl alkyl group; an amino group; or C₁ to C₅ alkyl group substituted bya hydroxyl, a carboxyl (—COOH) or a phosphonic acid (—PO₃MM′) group, Mand M′ may be the same or different and are each selected from the groupconsisting of a hydrogen atom, sodium, potassium, and ammonium (NH₄),and m and n are each 0 or an integer of 1 to 5;

wherein X² is —CH₂—, —CH(OH)—, —C(CH₃)(OH)—, —CH(COOM)-, or—C(CH₃)(COOM)—; and

wherein X³ to X⁷ are each independently a hydrogen atom; a C₁ to C₅alkyl group; an aryl group; an aryl alkyl group; an amino group; or C₁to C₅ alkyl group substituted by a hydroxyl, a carboxyl (—COOH) or aphosphonic acid (—PO₃H₂) group, provided that at least two of X³ to X⁷are a phosphonic acid group (—PO₃H₂), and m and n are each 0 or aninteger of 1 to 5.

In the formulas (I) to (III), the C₁ to C₅ alkyl group may have astraight or branched chain, including, for example, a methyl group, anethyl group, a propyl group, an isopropyl group, a butyl group, anisobutyl group, and a sec-butyl group. The aryl group may include, forexample, a phenyl group and a naphthyl group. The aryl alkyl group mayinclude any combination of the above alkyl group and the above arylgroup.

Examples of the complexing agent having the structure of formula (III)include but are not limited to aminotrimethylenephosphonic acid,1-hydroxyethylidene-1,1-diphosphonic acid,ethylenediaminetetramethylenephosphonic acid,diethylenetriaminepentamethylenephosphonic acid, or sodium, potassium orammonium salts thereof.

The inorganic phosphoric acids include but are not limited toorthophosphoric acid, pyrophosphoric acid, and tripolyphosphoric acid.Further, the salts thereof include but are not limited to alkali saltssuch as sodium salts and potassium salts, and ammonium salts.

One type or a mixture of two or more types of the complexing agents maybe used in the present invention. In the present invention, the amountof the complexing agent in the electroless displacement gold platingsolution can be appropriately determined depending on the undercoatmetal to be plated, the target thickness of the gold coating film, thewater-soluble gold compound, the water-soluble silver compound and thewater-soluble metal compound contained in the plating solution, and thelike. The amount of the complexing agent in the plating solution isgenerally from 0.01 to 2.0 mol/liter, preferably from 0.1 to 1.0mol/liter, more preferably from 0.5 to 0.7 mol/liter.

Any water-soluble, silver element-containing compound can be used as thewater-soluble silver compound for use in the electroless displacementgold plating solution of the present invention. The type of thewater-soluble silver compound used in the invention is appropriatelyselected depending on the undercoat metal to be plated, the desiredthickness of the gold coating film, the water-soluble gold compound andthe water-soluble metal compound contained in the plating solution, andthe like. Preferred water-soluble silver compounds include but are notlimited to potassium dicyanoargentate(I), silver oxide, silver nitrate,silver sulfate, and silver chloride.

The concentration of the water-soluble silver compound in theelectroless displacement gold plating solution is in the range of from1×10⁻⁶ to 1×10⁻³ mol/liter, preferably from 1×10⁻⁵ to 1×10⁻⁴ mol/liter,as a silver element.

One type or a mixture of two or more types of the water-soluble silvercompounds may be used.

The electroless displacement gold plating solution of the invention caninclude another water-soluble metal compound containing a metal elementother than a silver element and a gold element. Examples of thewater-soluble metal compound include a water-soluble thallium compound,a water-soluble lead compound, a water-soluble copper compound, and awater-soluble nickel compound. The type and the amount of thewater-soluble metal compound contained in the electroless displacementgold plating solution of the invention are appropriately determineddepending on the undercoat metal to be plated, the desired thickness ofthe gold coating film, the water-soluble gold compound and otheradditives contained in the plating solution, and the like.

Any water-soluble, thallium element-containing compound as be used asthe water-soluble thallium compound. Examples of the water-solublethallium compound include but are not limited to thallium cyanide,thallium sulfate, thallium nitrate, thallium chloride, thalliumcarbonate, thallium hydroxide, and thallium oxide. Preferredwater-soluble thallium compounds include thallium sulfate, thalliumnitrate, and thallium chloride. Further, the amount of the water-solublethallium compound in the electroless displacement gold plating solutionof the invention is in the range of from 5×10⁻⁶ to 2×10⁻³ mol/liter,preferably from 5×10⁻⁵ to 5×10⁻⁴ mol/liter as a thallium element.Furthermore, the water-soluble thallium compound is contained in theplating solution such that, with respect to the silver element containedin the plating solution, a molar ratio of the silver element to thethallium element is in the range of from 1:2000 to 200:1, preferablyfrom 1:50 to 2:1.

Any water-soluble, lead element-containing compound can be used as thewater-soluble lead compound. Examples of the water-soluble lead compoundinclude but are not limited to lead nitrate, lead hydroxide, leadchloride, lead phosphate, lead acetate, lead thiocyanate, and leadcyanide. Preferred water-soluble lead compounds include lead nitrate,lead hydroxide, and lead chloride. The amount of the water-soluble leadcompound in the electroless displacement gold plating solution of theinvention is in the range of from 5×10⁻⁷ to 5×10⁻⁴ mol/liter, preferablyfrom 5×10⁻⁶ to 5×10⁻⁵ mol/liter, as a lead element. The water-solublelead compound is contained in the plating solution such that, withrespect to the silver element contained in the plating solution, a molarratio of the silver element to the lead element is in the range of from1:500 to 2000:1, preferably from 1:5 to 20:1.

Any water-soluble, copper element-containing compound can be used as thewater-soluble copper compound. Examples of the water-soluble coppercompound include but are not limited to copper sulfate, copper nitrate,copper chloride, copper bromide, copper oxide, copper hydroxide, andcopper cyanide. Preferred water-soluble copper compounds include coppersulfate, copper nitrate, and copper chloride. Further, the amount of thewater-soluble copper compound in the electroless displacement goldplating solution of the invention is in the range of from 2×10⁻⁶ to2×10⁻³ mol/liter, preferably from 2×10⁻⁵ to 2×10⁻⁴ mol/liter, as acopper element. Furthermore, the water-soluble copper compound iscontained in the plating solution such that, with respect to the silverelement contained in the plating solution, a molar ratio of the silverelement to the copper element is in the range of from 1:2000 to 500:1,preferably from 1:20 to 5:1.

Any water-soluble, nickel element-containing compound can be used as thewater-soluble nickel compound. Examples of the water-soluble nickelcompound include but are not limited to nickel sulfate, nickel nitrate,nickel chloride, nickel hydroxide, nickel oxide, nickel fluoride, andnickel bromide. Preferred water-soluble nickel compounds include nickelsulfate, nickel nitrate, and nickel chloride. Further, the amount of thewater-soluble nickel compound in the electroless displacement goldplating solution of the invention is in the range of from 2×10⁻⁵ to2×10⁻² mol/liter, preferably from 2×10⁻⁴ to 2×10⁻³ mol/liter, as anickel element. In addition, the water-soluble nickel compound iscontained in the plating solution such that, with respect to the silverelement contained in the plating solution, a molar ratio of the silverelement to the nickel element is in the range of from 1:20000 to 50:1,preferably from 1:200 to 1:2.

One type or a mixture of two or more types of the water-soluble metalcompounds may be used in the present invention. If two or more types ofthe water-soluble metal compounds are used, the metals contained in therespective water-soluble compounds may be the same or different.

The pH of the electroless displacement gold plating solution of theinvention is appropriately adjusted depending on the undercoat metal tobe plated, the desired thickness of the gold coating film, thewater-soluble gold compound, the water-soluble silver compound and thewater-soluble metal compound contained in the plating solution, and thelike. From the viewpoint of prevention of the deterioration of theundercoat metal, the pH is preferably 11 or below, more preferably 10 orbelow, and still more preferably 7 or below.

For the above-mentioned pH adjustment, any pH adjuster can be used,including water-soluble acids and water-soluble bases. Examples of thepH adjuster include but are not limited to sodium hydroxide, potassiumhydroxide, ammonium hydroxide, sulfuric acid, sulfurous acid,hydrochloric acid, phosphoric acid, sulfamic acid, organic sulfonicacids, phosphonic acids, and carboxylic acids. Additionally, ifnecessary, any pH stabilizer may be added to the electrolessdisplacement gold plating solution of the invention in order to suppressthe fluctuation of the pH in the plating process. Applicable examples ofthe pH stabilizer include but are not limited to phosphates, phosphites,borates, and salts of carboxylic acids. The content of the pH stabilizerin the electroless displacement gold plating solution of the inventionis appropriately determined depending on the pH of the plating solutionand the other various compounds in the plating solution, and theconcentration is generally from 0.01 to 2 mol/liter, preferably from 0.1to 1 mol/liter.

For the purpose of improvement of the wettability on the metallicsubstrate to be plated, any wetting agent may be added to theelectroless displacement gold plating solution of the invention. Variousmaterials as far as they have been used for known gold plating processescan be used as the wetting agent, being not limited thereto. Examples ofthe wetting agent include but are not limited to nonionic surfactantssuch as polyoxyalkylene alkyl ether, polyoxyalkylene alkyl phenyl ether,polyoxyethylene polyoxypropylene glycol, fatty acid polyalkylene glycol,fatty acid polyalkylenesorbitans, and fatty acid alkanolamide; anionicsurfactants such as aliphatic carboxylates, alkanesulfonates,alkylbenzensulfonates, alkylnaphthalenesulfonates, alkylsulfates,polyoxyalkylene alkyl ether sulfates, alkylphosphates, polyoxyalkylenealkyl ether phosphates, and polyoxyalkylene alkyl phenyl etherphosphates; cationic surfactants such as alkylamine salts and quaternaryammonium salts; and amphoteric surfactants such as alkylbetaine,alkylimidazoline derivatives, and alkyldiethylenetriaminoacetic acid.The content of the wetting agent in the electroless displacement goldplating solution of the invention is appropriately determined dependingon the composition of the plating solution, the type of the metallicsubstrate, and the like, and the concentration is generally from 1×10⁻⁸to 1×10⁻² mol/liter, preferably from 1×10⁻⁶ to 1×10⁻⁴ mol/liter.

Any gold ion stabilizer for maintaining the stability of the gold ionsmay be added to the electroless displacement gold plating solution ofthe invention. The gold ion stabilizer includes but is not limited tocyanides such as potassium cyanide, sodium cyanide, and ammoniumcyanide; and substances capable of supplying sulfurous ion such assodium sulfite, potassium sulfite, and ammonium sulfite. Theconcentration of the stabilizer in the electroless displacement goldplating solution of the invention can be appropriately determineddepending on the content of the gold element at a concentration requiredfor forming the complex or at an excessive content to stabilize the goldcomplex. The concentration is generally from 2×10⁻⁴ to 0.5 mol/liter,preferably from 2×10⁻³ to 5×10⁻³ mol/liter.

The additive for use in preparing the electroless displacement goldplating solution of the invention includes a water-soluble silvercompound, a water-soluble thallium compound, a water-soluble leadcompound, a water-soluble copper compound, or a water-soluble nickelcompound. The additive according to the present invention may includeone type or two or more types of such water-soluble metal compounds. Iftwo or more types of the water-soluble metal compounds are contained,the respective water-soluble compounds may contain the same metal ordifferent metals.

The electroless displacement gold plating solution according to thepresent invention can be prepared by adding the additive containingrequired elements to any electroless displacement gold plating solution.For example, in the case where the additive of the invention containsonly the water-soluble silver compound, the additive is added to theelectroless displacement gold plating solution to prepare a silverelement-containing electroless displacement gold plating solution of theinvention. Alternatively, when the additive of the invention containsthe water-soluble silver compound and the water-soluble thalliumcompound, the additive is added to the electroless displacement goldplating solution to prepare an electroless displacement gold platingsolution containing the silver and thallium elements. In addition, thesilver and thallium containing electroless displacement gold platingsolution of the invention can also be prepared by adding two types ofthe additives of the invention: one including only the water-solublesilver compound; the other including only the thallium water-solublecompound, to any electroless displacement gold plating solution.

The water-soluble gold compound, water-soluble silver compound,water-soluble thallium compound, water-soluble lead compound,water-soluble copper compound, and water-soluble nickel compound usablefor the additive of the invention are the same as those described ascomponents of the electroless displacement gold plating solution of thepresent invention.

When the additive of the invention contains the water-soluble silvercompound and the water-soluble thallium compound, a molar ratio of thesilver element to the thallium element contained in the additive is inthe range of from 1:2000 to 200:1, preferably from 1:50 to 2:1. When theadditive of the invention contains the water-soluble silver compound andthe water-soluble lead compound, a molar ratio of the silver element tothe lead element contained in the additive is in the range of from 1:500to 2000:1, preferably from 1:5 to 20:1. When the additive of theinvention contains the water-soluble silver compound and thewater-soluble copper compound, a molar ratio of the silver element tothe copper element contained in the additive is in the range of from1:2000 to 500:1, preferably from 1:20 to 5:1. When the additive of theinvention contains the water-soluble silver compound and thewater-soluble nickel compound, a molar ratio of the silver element tothe nickel element contained in the additive is in the range of from1:20000 to 50:1, preferably from 1:200 to 2:1. In addition, when theadditive of the invention contains the silver element and a plurality ofmetal elements other than the silver element, each content of therespective metal elements other than the silver element contained in theadditive is set within each range for each element as described above.

The additive according to the present invention may further include acomplexing agent, a pH stabilizer, a pH adjuster, a wetting agent, or agold ion stabilizer, or any combination thereof, which are the same asthose described as components of the electroless displacement goldplating solution of the invention.

The additive according to the present invention can be in any form, asfar as it contains the above-described component(s). Examples of theform include but are not limited to solid, aqueous solution, dispersion,and suspension. Preferably, the additive is in the form of an aqueoussolution, because it can easily be mixed with any gold plating solution.

The electroless displacement gold plating solution according to thepresent invention can be prepared by adding the additive of theinvention to any electroless displacement gold plating solution. Theadditive is added in such an amount that each concentration of thesilver element, thallium element, lead element, copper element, nickelelement, the complexing agent, and the like fulfills the above-describedconcentration so as to composing the electroless displacement goldplating solution according to the present invention. Further, any knowngold plating solution containing a gold element can be used as anelectroless displacement gold plating solution to be added with theadditive of the present invention.

The additive according to the present invention is added to anyelectroless displacement gold plating solution, thereby improving thestability of the electroless displacement gold plating solution. Thestability of the plating solution means that, when a gold coating filmobtained by plating with a plating solution stored for a certain timeperiod from the preparation of the plating solution is compared withthat obtained by plating with a plating solution immediately after thepreparation of the plating solution, any one or both of the thickness ofthe gold plating film and evenness of the plated appearance aremaintained. Preferably, the characteristics are maintained even at oneweek, more preferably one month, and still more preferably one yearafter the preparation.

Electroless displacement gold plating is performed by treating ametallic substrate with the electroless displacement gold platingsolution of the invention, and thereby a gold coating film is formed ona surface of the metallic substrate. The metallic substrate for use inthe invention can be formed from any baser metal than gold. The metalmay be a metal comprising a gold element solely or an alloy comprising aplurality kinds of metal elements. Examples of the metal for thesubstrate include but are not limited to metals containing platinum,palladium, lead, silver, rhodium, copper, tin, iron, nickel, indium,cobalt, cadmium, chromium, zinc, aluminum, and titanium elements, andalloys thereof. The metals for use in the metallic substrate of theinvention may be metals containing nickel, cobalt, and palladiumelements, and alloys thereof. The metallic substrate may be a substrateobtained by thin electroless displacement gold plating to the metallicsubstrate comprising the above-mentioned metal or metals. Anyconventional process can be used as a thin electroless displacement goldplating process, for example, the process is performed by immersing themetallic substrate in any conventional thin electroless displacementgold plating solution.

The metallic substrate of the invention can be in any form. Examples ofsuch a form include but are not limited to plates such as flat platesand curved plates; bars; and balls. The metallic substrate may beprocessed to have fine structures such as trench and holes. Examples ofsuch substrates can include substrates for electronic parts such assubstrates for printed wiring boards and IC cards, ITO substrates, andsubstrates for ceramic IC packages. It is not necessary that in themetallic substrate of the invention, the whole substrate be constitutedof the above-described metal(s). The substrate may comprise a non-metalmaterial such as a ceramic or a resin, and a metal entirely or partiallycovering the surface of the non-metal material.

The treatment with the electroless displacement gold plating solution ofthe invention is achieved by bringing the metallic substrate intocontact with the plating solution. Any process can be used, as far as itcan bring the metallic substrate into contact with the plating solution.A preferred process is immersion of the metallic substrate into theplating solution. The treatment with the plating solution of theinvention is carried out at a plating temperature (plating solutiontemperature) of 50 to 95° C., preferably 60 to 90° C. When a platingtemperature is 50° C. or less, the deposition rate of the plated coatingis slow so that the productivity is uneconomically low. When a platingtemperature is above 95° C., the components in the plating solution maybe decomposed. The time period for the plating process in the presentinvention can be appropriately set depending on the desired thickness ofthe gold coating film, the metallic substrate used, and the like, beinggenerally 1 to 60 minutes, preferably 10 to 30 minutes.

In case of carrying out the electroless displacement gold platingtreatment according to the present invention, stirring of the platingsolution is permitted. Batch filtration or circulating filtration may becarried out. Particularly, the plating solution is preferably circulatedand filtrated with a filter, so that the temperature of the platingsolution can be even, and dusts, precipitates, and the like can beremoved from the plating solution. In addition, air may also beintroduced into the plating solution, so that the generation ofcolloidal gold particles or the precipitation of the generated goldparticles can effectively be prevented. The introduction of the air mayalso serves as the agitation of the plating solution with the air. Theair may also be blown into the plating solution aside from agitation.

The electroless displacement gold plating solution of the invention hasincreased stability so that the plating solution used in the electrolessdisplacement gold plating treatment of the invention may be oneimmediately after the preparation or after a lapse of a certain timeperiod from the preparation. The plating solution is used preferablywithin one month from the preparation, more preferably within one weekfrom the preparation, and still more preferably immediately after thepreparation.

In order to prevent the dilution of the components in the platingsolution, any pre-dipping step may be introduced prior to the treatmentof the metallic substrate with the gold plating solution of the presentinvention. The solution for the pre-dipping process herein is an aqueoussolution containing the above-mentioned complexing agent and/or thewater-soluble metal compound and being free of the gold element.

The electroless displacement gold plating treatment of the invention canprovide a metal composite material comprising a metallic substrate and agold coating film formed on the surface of the metallic substrate. Thegold coating film of the metal composite material has a thickness of0.10 μm or more, preferably 0.40 μm or more, and more preferably 0.50 μmor more. During a certain time period from the start of the plating tothe time when the displacement part of the metallic substrate has beenconsumed, the thickness of the gold coating film increases in accordancewith passage of time. As compared with conventional electrolessdisplacement gold plating treatment, the plating treatment using theelectroless displacement gold plating solution of the present inventionmay cause an increase in the coating thickness within a certain timeperiod, namely an increase of a plating rate. For example, with respectto a plating rate in case of using the plating solution of the presentinvention, an increase of 0.30 μm or more in the coating thickness per30 minutes from the start of the plating, preferably an increase of 0.40μm or more in the coating thickness per 30 minutes from the start of theplating, and more preferably an increase of 0.50 μm or more in thecoating thickness per 30 minutes from the start of the plating.

The gold coating film of the metal composite material of the presentinvention exhibits a even plated appearance. In the present invention,the plated appearance means a result of the visual test according to JISH 8617 for presence or absence of defects in the plated face. The evenplated appearance means that the characteristics such as brightness,dull deposits, and roughness are even in the plated face. An unevenplated appearance means that the plated face is uneven in thecharacteristics such as brightness, dull deposits, and roughness and hasstains, blisters, pits, flaws, or the like.

In the metal composite material of the present invention, an adhesion ofthe gold coating film to the metallic substrate is improved. Theadhesion can be determined by a tape test. The tape test is performedaccording to ASTM D-3359-95a. Specifically, in the tape test, ten cutlines in each of column and row are made and provide 1 mm square areaswith a knife, and a cellophane tape (NICHIBAN CO., LTD., 18 mm in width)is applied to the area and then peeled at a breath, and the peeledsquares are counted. The metal composite material of the presentinvention has preferably no peeled square.

The metal composite material obtained by the treatment of the metallicsubstrate with the electroless displacement gold plating solution of theinvention has a thick gold coating film, exhibits a even platedappearance, and is excellent in adhesion between the gold coating filmand the metallic substrate as describer above. Accordingly, adverseeffects on the physical properties of the coating such as wirebondability, solder jointability, and solder wettability caused byconventional processes can be reduced. Therefore, the plating method ofthe present invention is particularly suited for manufacturingelectronic parts in which the physical properties of the coating filmare required.

The metal composite material of the present invention is prepared by thetreatment of the metallic substrate with the electroless displacementgold plating solution containing silver, thallium, lead, copper, ornickel element. In the gold coating film of the composite material,therefore, the silver, thallium, lead, copper, or nickel element iscontained. The total content of the silver, thallium, lead, copper, andnickel elements in the gold coating film in the metal composite materialof the invention can be 0.5 mol % or less, preferably 0.01 mol % orless, based on the gold element.

EXAMPLES

Examples 1 to 8 described below each provide the electrolessdisplacement gold plating solution according to the present invention,and Comparative Examples 1 to 4 provide electroless displacement goldplating solutions which do not correspond to the present invention. Inpreparing each plating solution, each compound was dissolved in purewater, and pH was adjusted by potassium hydroxide such that therespective plating solutions had the following compositions. Thecompounds for each plating solution were reagent grade commerciallyavailable chemicals.

Example 1

Potassium Dicyanoaurate(I)   4 g/L (as gold element) OrthophosphoricAcid   1 mol/L Citric Acid 0.5 mol/L Patassium Dicyanoargentate(I)   1mg/L (as silver element) pH 6.0

Example 2

Potassium Dicyanoaurate(I)   4 g/L (as gold element) OrthophosphoricAcid   1 mol/L Citric Acid 0.5 mol/L Silver Oxide   1 mg/L (as silverelement) Thallium Sulfate  50 mg/L (as thallium element) pH 6.0

Example 3

Potassium Dicyanoaurate(I)   4 g/L (as gold element) OrthophosphoricAcid   1 mol/L Citric Acid 0.5 mol/L Potassium Silver Cyanide   1 mg/L(as silver element) Thallium Sulfate  50 mg/L (as thallium element) pH6.0

Example 4

Potassium Dicyanoaurate(I)   4 g/L (as gold element) OrthophosphoricAcid   1 mol/L Citric Acid 0.5 mol/L Potassium Silver Cyanide   1 mg/L(as silver element) Lead Nitrate   1 mg/L (as lead element) pH 6.0

Example 5

Potassium Dicyanoaurate(I)   4 g/L (as gold element) OrthophosphoricAcid   1 mol/L Citric Acid 0.5 mol/L Potassium Silver Cyanide   1 mg/L(as silver element) Copper Nitrate   1 mg/L (as copper element) pH 6.0

Example 6

Potassium Dicyanoaurate(I)   4 g/L (as gold element) OrthophosphoricAcid   1 mol/L Citric Acid 0.5 mol/L Potassium Silver Cyanide   1 mg/L(as silver element) Nickel Sulfate  10 mg/L (as nickel element) pH 6.0

Example 7

Potassium Dicyanoaurate(I)   4 g/L (as gold element) Iminodiacetic Acid0.5 mol/L Malic Acid 0.5 mol/L Potassium Silver Cyanide   1 mg/L (assilver element) Thallium Sulfate  50 mg/L (as thallium element) pH 6.0

Example 8

Sodium Tetrachloroaurate(III)   4 g/L (as gold element) OrthophosphoricAcid   1 mol/L Citric Acid 0.5 mol/L Silver Nitrate   1 mg/L (as silverelement) Thallium Sulfate  50 mg/L (as thallium element) pH 6.0

Comparative Example 1 (plating solution obtained by removing potassiumsilver cyanide from that in Example 1) Potassium Dicyanoaurate(I)   6g/L Orthophosphoric Acid   1 mol/L Citric Acid 0.5 mol/L pH 6.0

Comparative Example 2 (plating solution obtained by removing PotassiumDicyanoaurate(I)   6 g/L Orthophosphoric Acid   1 mol/L Citric Acid 0.5mol/L Thallium Sulfate  50 mg/L (as thallium element) pH 6.0

Comparative Example 3 (conventional displacement gold plating solution)Potassium Dicyanoaurate(I)   6 g/L Orthophosphoric Acid   1 mol/LEthylenediaminetetraacetic Acid 0.5 mol/L Thallium Sulfate  50 mg/L (asthallium element) pH 4.5

Comparative Example 4 (plating solution obtained by removing silvernitrate from that in Example 8) Sodium Tetrachloroaurate(III)   4 g/L(as gold element) Orthophosphoric Acid   1 mol/L Citric Acid 0.5 mol/LThallium Sulfate  50 mg/L (as thallium element) pH 6.0

The electroless displacement gold plating treatment of a metallicsubstrate was carried out as follows.

A 4×4 cm copper plate was electroless plated with nickel about 5 μm inthickness by a conventional process and then thin-plated with gold about0.03 μm in thickness by displacement plating. The resulting plate wasthick-plated with gold by treating it with each electroless displacementgold plating solution of Examples 1 to 8 and Comparative Examples 1 to 4at 85° C. for-30 minutes. The resulting test piece thick-plated withgold by displacement plating was measured for the thickness of the goldcoating film with a fluorescent X-ray microthicknessmeter (SeikoInstruments Inc.). The test piece thick-plated with gold by displacementplating was also visually observed for its appearance according to JIS H8617. Each electroless displacement gold plating solution of Examples 1to 8 and Comparative Examples 1 to 4 was used immediately after thepreparation and one week after the preparation. The results are shown inTable 1. TABLE 1 Results of thickness measurement of displacement goldplating deposition and results of plated appearance observationImmediately After One Week After Preparation Preparation DepositedDeposited Gold Gold Thickness Plated Thickness Plated Type of Bath (μm)Appearance (μm) Appearance Example 1 0.457 even 0.448 even Example 20.466 even 0.456 even Example 3 0.404 even 0.412 even Example 4 0.444even 0.455 even Example 5 0.439 even 0.447 even Example 6 0.460 even0.470 even Example 7 0.501 even 0.512 even Example 8 0.401 even 0.408even Comparative 0.173 even 0.134 even Example 1 Comparative 0.303 even0.252 uneven Example 2 Comparative 0.422 even 0.301 uneven Example 3Comparative 0.210 uneven 0.167 uneven Example 4

As shown in Table 1, when the plating solutions of Examples 1 to 8 whichare the electroless displacement gold plating solution according to thepresent invention are used, even one week after the preparation as wellas immediately after the preparation, a even plated appearance can beexhibited, and the thickness of each gold coating film is 0.40 μm ormore. Therefore, it is found that a sufficiently thick-plated goldplating is possible and the stability of the plating solution isexcellent. On the contrary, in Comparative Examples 1 to 4 in which theelectroless displacement gold plating solution of the invention is notused, both of a even plated appearance and a sufficient thickness of thegold coating film can not be obtained at the same time. In particular,one week after the preparation of the plating solution, a even platedappearance and a sufficient thickness of the gold coating film cannot beobtained. Therefore it is found that the stability of the platingsolution is insufficient.

INDUSTRIAL APPLICABILITY

As described above, the electroless displacement gold plating solutionaccording to the present invention contains a predetermined amount of asilver element or a combination of a silver element and at least oneelement selected from thallium, lead, copper, and nickel. This providesadvantages that, in a gold coating film of a metal composite materialobtained by treating a metallic substrate with the plating solution, aplated appearance is even, a thickness of the gold coating film issufficient, a plating speed is high, an adhesion of the gold coatingfilm to the undercoat metal is excellent, and further there is noadverse effect on the properties of the coating such as wirebondability, solder jointability, and solder wettability. Further, theelectroless displacement gold plating solution of the invention has anexcellent stability so that the plating treatment is possible regardlessof time after the preparation. In addition, the additive containingsilver element and/or thallium, lead, copper, or nickel element, whichshould be contained in the electroless displacement gold platingsolution, at a predetermined content facilitates the preparation of theelectroless displacement gold plating solution.

1. An electroless displacement gold plating solution comprising: 1) atleast one water-soluble gold compound providing from 5×10⁻⁴ to 5×10⁻²mol/liter of a gold element; 2) 0.01 to 2.0 mol/liter of at least onecompleting agent; and 3) at least one water-soluble silver compoundproviding from 1×10⁻⁶ to 1×10⁻³ mol/liter of a silver element.
 2. Theelectroless displacement gold plating solution according to claim 1,further comprising a water-soluble metal compound selected from thegroup consisting of at least one water-soluble thallium compoundproviding from 5×10⁻⁶ to 2×10⁻³ mol/liter of a thallium element, atleast one water-soluble lead compound providing from 5×10⁻⁷ to 5×10⁻⁴mol/liter of a lead element content, at least one water-soluble coppercompound providing from 2×10⁻⁶ to 2×10⁻³ mol/liter of a copper element,at least one water-soluble nickel compound providing from 2×10⁻⁵ to2×10⁻² mol/liter of a nickel element, and combinations thereof.
 3. Theelectroless displacement gold plating solution according to claim 2,wherein a molar ratio of the silver element to the thallium element isin the range of from 1:2000 to 200:1, a molar ratio of the silverelement to the lead element is in the range of from 1:500 to 2000:1, amolar ratio of the silver element to the copper element is in the rangeof from 1:2000 to 500:1, and a molar ratio of the silver element to thenickel element is in the range of from 1:20000 to 50:1.
 4. Theelectroless displacement gold plating solution according to any one ofclaims 1 to 3, wherein the solution has a pH of 10.0 or less.
 5. Anadditive for use in preparing the electroless displacement gold platingsolution according to any one of claims 1 to 4, comprising awater-soluble silver compound, a water-soluble thallium compound, awater-soluble lead compound, a water-soluble copper compound, or awater-soluble nickel compound, or any combination thereof.
 6. Anadditive for use in preparing the electroless displacement gold platingsolution according to any one of claims 2 to 4, comprising awater-soluble silver compound and a water-soluble metal compound,wherein the water-soluble metal compound is selected from the groupconsisting of a water-soluble thallium compound, a water-soluble leadcompound, a water-soluble copper compound, and a water-soluble nickelcompound, and combinations thereof, and in the additive, a molar ratioof the silver element to the thallium element is in the range of from1:1500 to 1:2, a molar ratio of the silver element to the lead elementis in the range of from 1:500 to 2000:1, a molar ratio of the silverelement to the copper element is in the range of from 1:2000 to 500:1,and a molar ratio of the silver element to the nickel element is in therange of from 1:2000 to 500:1.
 7. The additive for use in preparing theelectroless displacement gold plating solution according claim 5 to 6,further comprising a complexing agent, a pH stabilizer, a pH adjuster, awetting agent, or a gold ion stabilizer, or any combination thereof. 8.An electroless displacement gold plating method, comprising treating ametallic substrate with the electroless displacement gold platingsolution according to any one of claims 1 to
 4. 9. The electrolessdisplacement gold plating method according to claim 8, wherein themetallic substrate is thin-plated with gold by displacement plating. 10.A metal composite material prepared by the method according to claim 8or 9, having a gold coating film on a surface of the metallic substrate.11. The metal composite material according to claim 10, wherein athickness of the gold coating film is improved.
 12. The metal compositematerial according to claim 10, wherein a plating appearance of the goldcoating film is improved.
 13. The metal composite material according toclaim 10, wherein an adhesion of the gold coating film is improved. 14.The metal composite material according to claim 11, wherein the goldcoating film has a thickness of 0.40 μm or more.
 15. A method forstabilizing an electroless displacement gold plating solution,comprising: adding the additive for use in preparing the electrolessdisplacement gold plating solution according to anyone of claims 5 to 7to an electroless displacement gold plating solution.